Northern California residents could feel magnitude 6.0 aftershocks for months following Thursday morning’s magnitude 7.0 earthquake off the coast of Eureka, California.

The main earthquake struck around 10:45 a.m. local time off the coast of Cape Mendocino. At least 34 aftershocks also occurred in the region in the following hours. The strongest aftershock was a magnitude 4.3 quake that struck near Petrolia, California, around 12:04 p.m. local time, according to Tim Clements, a geophysicist at the US Geological Survey (USGS) Earthquake Science Center.

“We expect aftershocks to occur in the area for weeks to months,” Clements said Newsweek. “According to our aftershock forecast, the chance of an M6+ earthquake this week is about 1 in 20,” or 5 percent.

Aftershocks are smaller earthquakes that follow a larger earthquake. According to a USGS map, many of the aftershocks felt Thursday originated offshore, but some also occurred inland.

According to the USGS aftershock forecast, milder aftershocks are more likely to occur.

There is a greater than 99 percent chance that additional earthquakes of magnitude 3.0 or greater will occur. The forecast assumes an 84 percent chance of quakes of magnitude 4.0 or greater, a 31 percent chance of quakes of magnitude 5.0 and a less than 1 percent chance of earthquakes of magnitude 7.0.

According to the USGS, damage typically doesn’t occur until an earthquake is around magnitude 4 or 5.

The earthquake, which occurred off the coast of northwestern California, occurred in the Mendocino Triple Junction, likely near the Mendocino rupture zone, Clements said.

“The Mendocino Triple Junction is where the Gorda, North America and Pacific plates meet,” he said. “This area has experienced several M6+ earthquakes over the past decade,” including an M6.4 earthquake on December 20, 2022, an M6.2 earthquake on December 20, 2021, and an M6.6 earthquake on December 8, 2016.

The last time an earthquake of magnitude 7.0 or greater struck the area was a magnitude 7.2 earthquake on April 25, 1992. Eight earthquakes of magnitude 7.0 or greater have struck the area since 1922, including the one on Thursday.

This quake was strong enough to trigger a tsunami warning for about 600 miles of coastline in California and Oregon, but no tsunami occurred.

Clements advised people in the affected area to check buildings and structures for possible damage before re-occupying them.

If strong shaking occurs due to aftershocks, the USGS recommends that people “throw, cover and hold on.”

Residents said they felt tremors in populated areas such as Sacramento, Eureka and Monterey Los Angeles Times. However, Eureka city officials said no major damage was reported hours after the quake, although officials had little time to assess the damage before evacuating due to the tsunami warning.